A comparator can generally operate in various stages, such as an input stage to an output stage in sync with a single clock signal. The comparator is generally realized by an amplifier or a latch. One important condition required in the comparator is to eliminate a hysteresis phenomenon in which the state of the current period of the clock signal is forced to be maintained. In this regard, the comparator uses a switch to connect differential outputs, and thus, resets the output of the comparator, for every period of the clock signal. However, switching carried out when latching is begun after output reset may generate a kickback phenomenon in a circuit to drive the comparator. In particular, such a phenomenon inevitably occurs in a single-stage comparator. In this case, there may be direct adverse affect on accuracy.
A comparator compares an input voltage with a reference voltage, amplifies a difference between the input voltage and the reference voltage as the result of the comparison, and outputs the result of the comparison, which has a “high” or “low” logic level. Since the comparator does not have a noise compensation function, it additionally uses a separate analog or digital compensation circuit. As a circuit to solve a noise problem, a Schmitt trigger circuit may be added to the comparator. However, the Schmitt trigger circuit has a drawback in that it is sensitive to a process variation upon determining a positive threshold voltage Vth+ and a negative threshold voltage Vth−, due to characteristics thereof. For this reason, recently-developed comparators themselves are designed to have hysteresis characteristics.
In such a comparator, which has hysteresis characteristics, there is no variation in output even when a voltage input to the comparator is lowered under the condition that the output is maintained at a “high” level while the difference of the voltage from another voltage input to the comparator is “0.” When the input voltage, which is continuously lowered, reaches a lower reference voltage, the output is transited from the “high” level to a “low” level. Even when the input voltage rises in this state, the output is maintained at the “low” level. However, when the input voltage reaches an upper reference voltage as it rises continuously, it is transited from the “low” level to the “high” level. Here, hysteresis characteristics mean there are two input voltage points where the output voltage is varied, namely, two input voltage points respectively corresponding to the upper and lower reference voltages.
In order to exhibit high resistance against noise in the above-mentioned comparator having hysteresis characteristics, however, errors may occur in the comparator itself when the hysteresis characteristics of the comparator are varied in accordance with process variation. In this case, there may be a problem in the operation reliability of the entirety of a semiconductor device to which the comparator is applied.
Meanwhile, the comparator may be used in a voltage regulator. A low dropout (LDO) regulator is a linear regulator. The linear regulator is adapted to generate a desired output voltage by eliminating an excessive input voltage using a transistor operating within a linear range. Recently-developed portable appliances require a miniature size, light weight, and long charge life. In order to achieve a long charge life, it is necessary to maximize the use of limited supply power. An effective method capable of solving this problem is to reduce net power consumption.
Meanwhile, various voltages are used in a system. A baseband circuit, an analog to digital converter (ADC), and a digital to analog converter (DAC), which are used in encoder and decoder blocks for data processing, and a spreader and a despreader, which are used for spread spectrum radio frequency (RF) communication, operate at 1.2V, in order to minimize power loss. The voltage output generated by an LDO regulator in a system on chip (SoC) should be supplied to an external appliance under the condition that the voltage output is sufficiently stable. Since the output voltage from the LDO regulator used as a voltage source in the external appliance, malfunction may occur in the external appliance when the output voltage from the LDO regulator is applied to the circuit of the external appliance before the output voltage is converged at a stable level.
In this regard, an error amplifier is used to compare the output voltage from the LDO regulator with a reference voltage, in order to converge the output voltage from the LDO regulator at a normal value. However, when the reference voltage is swung, or the output voltage from the LDO regulator is momentarily swung. there may be a problem in that the error amplifier may output a value of “1” when a value of “0” should be output, or may output a value of “0” when a value of “1” should be output.